Technical Field
The present invention relates to a method for producing a polyacrylic acid (salt)-based water-absorbing resin.
Related Art
A water-absorbing resin (SAP/Super Absorbent Polymer) is a polymer gelling agent having water swellable property and water insoluble property and has been widely used as absorbing agents for hygiene products such as disposable diapers and sanitary napkins, agriculture/horticulture water retention agents, industrial waterproofing materials and the like.
The water-absorbing resin is produced from various kinds of monomers or hydrophilic polymers as a raw material. In particular, a polyacrylic acid (salt)-based water-absorbing resin using as a monomer an acrylic acid and/or a salt thereof has high absorbent performance, and thus most widely produced industrially.
The water-absorbing resin is produced as a particulate product through various steps such as polymerization, drying, crushing, classification, and surface crosslinking (Non-Patent Literature 1). Further, as disposable diapers, a primary application of the water-absorbing resin, become more high-performance, more functionalities (physical properties) are needed. Specifically, in addition to fluid retention capacity, examples of such functionalities (physical properties) include gel strength, water soluble component, water absorption speed, fluid retention capacity under pressure, liquid permeability, particle size distribution, urine resistance, antibacterial property, impact resistance (damage resistance), powder fluidity, deodorant property, anti-coloring property (whiteness), and low dustiness.
Addition of an additive is mentioned as a method for providing these functions (physical properties).
As a technique of adding an additive, there are techniques of adding an additive to a polymerization reaction vessel using a screw conveyor (Patent Literatures 1 and 2). In addition, techniques of adding a reducing agent or an oxidizer in order to reduce residual monomers (Patent Literatures 3 and 4) are proposed.
Furthermore, among the aforementioned functions (physical properties), water absorption speed is considered particularly important, and as a technique of improving the water absorption speed, foaming polymerization and the like (Patent Literatures 5 to 9) are proposed.